In the past, the “board test coverage” provided by a particular test suite was often measured in terms of “device coverage” and “shorts coverage”. Device coverage was measured as the percentage of board devices with working tests, and shorts coverage was measured as the percentage of accessible board nodes.             Device      ⁢                           ⁢      Coverage        =                  #        ⁢                                   ⁢        Tested        ⁢                                   ⁢        Devices                    Total        ⁢                                   ⁢        #        ⁢                                   ⁢        of        ⁢                                   ⁢        Devices                        Shorts      ⁢                           ⁢      Coverage        =                  #        ⁢                                   ⁢        Accessible        ⁢                                   ⁢        Nodes                    Total        ⁢                                   ⁢        #        ⁢                                   ⁢        of        ⁢                                   ⁢        Nodes            
The above model of board test coverage was developed at a time when testers had full nodal access to a board (i.e., access to the majority (typically 95-100%) of a board's nodes). Boards were also less dense, less complex, and somewhat more forgiving due to their lower frequency of operation. In this environment, the above model was acceptable.
Over the last decade, boards have migrated towards limited access. In fact, it is anticipated that boards with access to less than 20% of their nodes will soon be common. Some drivers of access limitation include:                Increasing board density (devices/square centimeter is increasing)        Fine line and space geometry in board layouts (i.e., smaller probe targets)        Grid array devices of increasing pitch density        High-frequency signals that demand precise layouts and offer no probe targets        Board node counts that are several times greater than the maximum available on any tester        
The above changes have made application of the “old” model of board test coverage difficult at best, and meaningless in many cases.
Usefulness of the “old” model of board test coverage has also been impacted by the advent of new and radically different approaches to testing (e.g., Automated Optical Inspection (AOI) and Automated X-ray Inspection (AXI)). Many of the new test approaches are very good at testing for certain defects, but limited in terms of the number of defects they can test. Thus, more and more often, it is becoming erroneous to presume that a device with working tests is a sufficiently tested device. As a result, a board is often submitted to different test processes, which in combination define the “test suite” for a particular board (see FIG. 2).
Given the above state of characterizing board test coverage, new methods and apparatus for characterizing board test coverage are needed.